1. Field of the Invention
The present invention relates to a sheet processing apparatus, an image forming system, and a sheet-bundle additional folding method, and more particularly, to a sheet processing apparatus having a function of folding a sheet-like recording medium such as paper, recording paper, and transfer paper (hereinafter, referred to simply as a “sheet” in the present specification), an image forming system including the sheet processing apparatus, and a sheet-bundle additional folding method implemented by the sheet processing apparatus.
2. Description of the Related Art
Conventionally, in post-processing apparatuses used in combination with an image forming apparatus such as a copier, there are ones that bind a bundle of one or more sheets into a saddle-stitched booklet by saddle-stitching and folding the center of the sheet bundle with use of a pair of folding rollers installed parallel to each other in a sheet folding direction.
Furthermore, there is already known an additional folding technique for enhancing a fold in a folded saddle-stitched booklet by moving an additional folding roller along the fold after saddle-stitching and center-folding are performed on a sheet bundle.
As one of such additional folding techniques, for example, a technology disclosed in Japanese Patent Application Laid-open No. 2012-153530 is well known.
A sheet processing apparatus according to this technology includes a folding roller unit, an additional folding roller unit, and a drive unit. The folding roller unit makes a fold in a sheet while the sheet is passing through a nip between a pair of rollers. The additional folding roller unit includes first to third rollers: the first roller installed on the side of a first plane perpendicular to a direction of conveying a sheet bundle folded by the folding roller unit and the second and third rollers installed on the side of a second plane perpendicular to the sheet-bundle conveying direction; the second and third rollers each form a nip with the first roller. The drive unit drives the additional folding roller unit to move along the fold in a state where the sheet bundle is held in the nip between the first and second rollers and the nip between the first and third rollers. This configuration enables the sheet processing apparatus to perform additional folding sufficiently when performing the additional folding on a sheet bundle.
Specifically, the additional folding roller unit described in Patent document 1 includes three additional folding rollers, and is driven to move along a fold in a sheet bundle in a state where the fold is held between the rollers. In this regard, a roller having a larger diameter than those of the second and third rollers is used as the first roller.
In a case of using three folding rollers in this way, two nips are formed, and respective tangential angles of the nips are not parallel. Therefore, a direction of force applied to a staple differs between the nips, and therefore the staple may be deformed.
There is a method for enhancing a fold by shifting an angle between a direction of the tangent to a pair of additional folding rollers and a thickness direction of a sheet bundle from 90 degrees; however, in this enhancing method, a staple may be deformed depending on the position of the staple in the additional folding.
Namely, if we focus on enhancing a fold, a staple may be deformed; on the other hand, if we focus on preventing deformation of a staple, enhancement of a fold may be insufficient.
Therefore, there is a need for a sheet processing apparatus to enable a user to make a choice between emphasis on enhancement of a fold and emphasis on suppression of staple deformation in accordance with user's intention.